lib: packing: add pack_fields() and unpack_fields()

JIRA: https://issues.redhat.com/browse/RHEL-92666

Conflicts:
- adjusted context conflict in scripts/.gitignore due to missing
  c61452aaad6bb ("scripts: remove bin2c")

Upstream commit(s):
commit 41d7ea3
Author: Vladimir Oltean <vladimir.oltean@nxp.com>
Date:   Tue Dec 10 12:27:12 2024 -0800

    lib: packing: add pack_fields() and unpack_fields()

    This is new API which caters to the following requirements:

    - Pack or unpack a large number of fields to/from a buffer with a small
      code footprint. The current alternative is to open-code a large number
      of calls to pack() and unpack(), or to use packing() to reduce that
      number to half. But packing() is not const-correct.

    - Use unpacked numbers stored in variables smaller than u64. This
      reduces the rodata footprint of the stored field arrays.

    - Perform error checking at compile time, rather than runtime, and return
      void from the API functions. Because the C preprocessor can't generate
      variable length code (loops), this is a bit tricky to do with macros.

      To handle this, implement macros which sanity check the packed field
      definitions based on their size. Finally, a single macro with a chain of
      __builtin_choose_expr() is used to select the appropriate macros. We
      enforce the use of ascending or descending order to avoid O(N^2) scaling
      when checking for overlap. Note that the macros are written with care to
      ensure that the compilers can correctly evaluate the resulting code at
      compile time. In particular, care was taken with avoiding too many nested
      statement expressions. Nested statement expressions trip up some
      compilers, especially when passing down variables created in previous
      statement expressions.

      There are two key design choices intended to keep the overall macro code
      size small. First, the definition of each CHECK_PACKED_FIELDS_N macro is
      implemented recursively, by calling the N-1 macro. This avoids needing
      the code to repeat multiple times.

      Second, the CHECK_PACKED_FIELD macro enforces that the fields in the
      array are sorted in order. This allows checking for overlap only with
      neighboring fields, rather than the general overlap case where each field
      would need to be checked against other fields.

      The overlap checks use the first two fields to determine the order of the
      remaining fields, thus allowing either ascending or descending order.
      This enables drivers the flexibility to keep the fields ordered in which
      ever order most naturally fits their hardware design and its associated
      documentation.

      The CHECK_PACKED_FIELDS macro is directly called from within pack_fields
      and unpack_fields, ensuring that all drivers using the API receive the
      benefits of the compile-time checks. Users do not need to directly call
      any of the macros directly.

      The CHECK_PACKED_FIELDS and its helper macros CHECK_PACKED_FIELDS_(0..50)
      are generated using a simple C program in scripts/gen_packed_field_checks.c
      This program can be compiled on demand and executed to generate the
      macro code in include/linux/packing.h. This will aid in the event that a
      driver needs more than 50 fields. The generator can be updated with a new
      size, and used to update the packing.h header file. In practice, the ice
      driver will need to support 27 fields, and the sja1105 driver will need
      to support 0 fields. This on-demand generation avoids the need to modify
      Kbuild. We do not anticipate the maximum number of fields to grow very
      often.

    - Reduced rodata footprint for the storage of the packed field arrays.
      To that end, we have struct packed_field_u8 and packed_field_u16, which
      define the fields with the associated type. More can be added as
      needed (unlikely for now). On these types, the same generic pack_fields()
      and unpack_fields() API can be used, thanks to the new C11 _Generic()
      selection feature, which can call pack_fields_u8() or pack_fields_16(),
      depending on the type of the "fields" array - a simplistic form of
      polymorphism. It is evaluated at compile time which function will actually
      be called.

    Over time, packing() is expected to be completely replaced either with
    pack() or with pack_fields().

    Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
    Co-developed-by: Jacob Keller <jacob.e.keller@intel.com>
    Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
    Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
    Link: https://patch.msgid.link/20241210-packing-pack-fields-and-ice-implementation-v10-3-ee56a47479ac@intel.com
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: Petr Oros <poros@redhat.com>

Author: orosp

MAINTAINERS

@@ -14637,6 +14637,7 @@ F:	Documentation/core-api/packing.rst
 F:	include/linux/packing.h
 F:	lib/packing.c
 F:	lib/packing_test.c
+F:	scripts/gen_packed_field_checks.c
 
 PADATA PARALLEL EXECUTION MECHANISM
 M:	Steffen Klassert <steffen.klassert@secunet.com>

Makefile

@@ -1385,6 +1385,10 @@ PHONY += scripts_unifdef
 scripts_unifdef: scripts_basic
 	$(Q)$(MAKE) $(build)=scripts scripts/unifdef
 
+PHONY += scripts_gen_packed_field_checks
+scripts_gen_packed_field_checks: scripts_basic
+	$(Q)$(MAKE) $(build)=scripts scripts/gen_packed_field_checks
+
 # ---------------------------------------------------------------------------
 # Install